High-level soldiers are the best for this, as they're tougher, better armoured, better armed and better skilled. Many swear by Nord Huscarls for sieges - I've never been able to raise an appreciable force of them myself, so I can't say.
As for those who overcome a fully garrisoned city with 50-70 soldiers, I suspect they're either using decreased damage, or an extremely high-level character they've become very adept at handling. Or both. Either way, don't worry about replicating their feats just yet.
I play on 107% difficulty, full damage, full battlesize, etc. Personally, the closest I've come to taking a city with a small force has been slightly over a 1.3:1 attacker/defender ratio. I follow this strategy: Put as many marksmen or sharpshooters on the field as you can and have them hold ground in a position where they can swamp the walls with fire. Place your infantry in front of them as shields. Take a ranged weapon yourself and snipe any archers in positions flanking the siege tower or ladder. Once they're cleared out, send up your infantry in the first assault. Keep your own archers on the ground and firing. Once you break onto the walls, move your archers up onto the battlements to fire down on the enemies in the courtyard. They can also help keep the ladder secure if your infantry is pushed back.
I also suggest outnumbering the defenders if possible, as it will mean fewer enemies on the wall. Also, until you're very high-level, well-armoured and proficient in surviving close-quarters melee, let your soldiers do the brunt of the fighting. If they die, you just get reinforcements, but if you die, they retreat and you have to rebuild all your siege equipment.